This invention relates to a method for determining the preferred direction of rotation of a vehicle tire and more particularly to such a method whereby the tire may be mounted on a vehicle so that when the tire is rotating in the forward direction of the vehicle the forces acting on the tire, which tend to cause the vehicle to "pull" to one side, may be minimized.
It has long been noted that if an automobile is being driven in a straight path and the steering wheel is released, the vehicle will usually tend to pull or drift toward one side or the other of the straight path. It has been more recently noted that the problem is exacerbated by the utilization of modern, belted, radial ply tires, by which is meant radial ply carcass tires which are constructed with a breaker or belt interposed between the crown region of the tire carcass and the tire tread, the breaker being usually comprised of two layers or plies of tire cords or cables which are generally inextensible, similar but opposed low bias orientations of the cords or cables being employed in successive plies.
This problem of tire pull, which is hazardous due to the fatigue induced in the driver of the vehicle by the necessity of constantly utilizing force to maintain a straight path of vehicle travel and due to the fact that even a momentary release of the steering wheel may result in a loss of vehicle control, is caused by forces which the tire absorbs from the road surface over which the tire is traveling. The two forces here of primary interest, commonly referred to in the art as lateral structural force and self-aligning torque, each cause the tire, when in service on a vehicle and thus under load, to follow a path which is angularly offset from the median equatorial plane of the tire. The lateral structural force acting on the tire causes an angular offset or "slip angle" .alpha. s while the self-aligning torque acting on the tire causes an angular offset or slip angle .alpha. m, and it is the fact that these two slip angles, .alpha. s and .alpha. m, are generally unequal which causes the aforementioned problem of pull or drift.